Galvanic corrosion has played a significant role in history, particularly when it comes to naval warship development and advances in metallurgy. The fascinating history of galvanic corrosion includes accidental discovery, trial and error, and a rapidly deteriorating Statue of Liberty.
Today, galvanic corrosion has impacted the way in which modern warships and submarines are designed, how civil engineers plan and build infrastructure, and even how agricultural equipment is manufactured to stand the test of time.
When this scientific phenomenon was first discovered by accident in the 18th century, little was known about how devastating galvanic corrosion could be to vital metal components. With modern science and knowledge, we now fully understand why galvanic corrosion occurs, its ultimate effects and how to prevent it from happening.
Galvanic corrosion is a type of electrochemical corrosion that occurs when two dissimilar metals are in direct contact with each other and are exposed to an electrolyte, such as water. When this occurs, one of the metals (the anode) corrodes faster than it normally would, while the other metal (the cathode) is protected from corrosion. The metal that corrodes is usually the one that is less noble (more chemically reactive).
This process happens because the metals create a small electric current when they come into contact, causing electrons to flow between them and rapidly deteriorate the less resistant metal.
Galvanic corrosion is commonly seen in marine environments, where saltwater acts as the electrolyte, making it a big concern for ships, bridges, and oil platforms.
In 1761, the Royal Navy frigate HMS Alarm was deployed to the West Indies, fitted with experimental copper sheets below its waterline with the intention of combatting weed accumulation and the devastating effect of wood-boring worms.
This innovative solution proved highly effective, not only protecting the ship’s timber hull but also significantly reducing biofouling.
With hulls free of marine growth, British ships were able to out speed, outmanoeuvre and outgun Spanish and French ships in Caribbean waters.
However, these advantages were soon outweighed by the negatives when HMS Alarm returned to Britain. It was discovered that some of the copper sheets had detached from the hull due to the rapid deterioration of the iron nails that held the copper sheets to the timber.
Surprisingly, some of the iron nails remained undamaged while others had dissolved into a ‘kind of rusty Paste’. On closer inspection, the preserved nails still had pieces of their original oiled paper between the copper sheets and the iron nails (accidentally left by ship fitters rushing the installation).
These early observations sparked a series of experiments throughout the late 18th century. Various coatings and fasteners were explored, with shipbuilders ultimately settling on a copper-zinc alloy that provided the necessary strength.
By 1782, the Royal Navy had equipped 82 capital ships, 115 frigates, and 102 sloops with copper sheets.
These trials marked a pivotal moment in the understanding of galvanic corrosion. The Royal Navy’s experiences highlighted the importance of metal compatibility in saltwater environments and the potential benefits of electrical isolation.
It also highlighted the superior antifouling and antimicrobial properties of copper in a marine setting, pathing the way for modern cupro-alloys that leverage this remarkable property.
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In the 80’s, during routine maintenance checks of the Statue of Liberty, it was discovered that the iconic statue was suffering from galvanic corrosion problems. It was instantly evident that the cause was a reaction between the outer copper façade and the wrought iron internal support structure, accelerated by the salt-rich atmosphere of Liberty Island.
When it was designed and constructed by Gustave Eiffel in the 1880’s, shellac (a type of varnish) was used between the iron supports and the outer copper cladding. Unbeknownst to Eiffel, this had acted as an insulation barrier for nearly a century but was gradually broken down by the salty moisture of Liberty Island, and polluted rainwater. This combination promoted a galvanic interaction between the two metals. As iron is the least noble (and thus more reactive) of the two, it acted as a sacrificial anode and began to rust.
To combat this galvanic corrosion and ensure the Statue of Liberty remains standing for centuries to come, the original shellac insulation was replaced with PTFE.
The rate of galvanic corrosion depends on several different factors, which means there isn’t an accurate timeline of when galvanic corrosion occurs and deteriorates metal components. Factors that affect the speed of corrosion include:
- Types of metals
- Environment
- Electrolytes
- Anode size
- Surface area ratio of anode and cathode
There are ways to prevent galvanic corrosion or lessen its affects on metal components. You can try to prevent galvanic corrosion by:
- Using similar metals – Choose metals or alloys that have similar corrosion properties.
- Using an electrical insulator – By creating a non-conductive barrier between the two different metals, you can break the electrical connection and prevent galvanic corrosion.
- Applying a coating – Shield the metals from contact from electrolytes by using an alloy or non-metal coating.
- Using a spacer – Create separation between the two metals with a spacer made from another material.
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